microSD card capacity

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[ Date | 2020-12-14 22:02 -0500 ]
[ Mod. | 2020-12-21 13:11 -0500 ]
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I have those two Samsung microSD cards, nominally of the same model (MB-MC32D: 32GB Evo+ UHS-I, class 10).

Supposedly identical microSD cards

Supposedly identical microSD cards

The one on the left on the picture has the following capacity in bytes:

# blockdev --getsize64 /dev/mmcblk0 

The other card holds this many bytes:

# blockdev --getsize64 /dev/mmcblk0 

That1 is 220×3×101812^{20} \times 3 \times 10181 versus 226×32×532^{26} \times 3^{2} \times 53, respectively, or, if we retain the largest common power of two factor only, 220×305432^{20} \times 30543 vs 220×305282^{20} \times 30528: the first card is larger by precisely 15 megabytes2.

That capacity discrepancy was a bit unfortunate, as I intended to replace the card on the left, which was dying, and had recently turned itself read-only3, with the other one.

Follow-up: SanDisk

In late December, I finally received a new pair of replacement microSD cards (SanDisk Ultra microSDHC™ UHS-I "card with adapter, for better photos and full HD video, A1 for faster mobile app performance, 32GB, speed up to 98MB/s, 653x"; the front of the packaging bears a circled "class 10" logo, the back has SDHC I, microSDHC I, A1, and the card itself has that plus A1, microSDHC I, UHS-I, and the back of the packaging).

The card's capacity shows as:

# blockdev --getsize64 /dev/mmcblk0 

This is 219×3×103×1972^{19} \times 3 \times 103 \times 197 bytes.

Card Bytes Megabytes
Samsung C626 32026656768 30543
Samsung A652 32010928128 30528
SanDisk 31914983424 30436½

The newer SanDisk is smaller than the smallest of the two Samsung cards by 91½ megabytes.

  1. Cf. https://github.com/haguenau/cute-factor

  2. I mean real megabytes here, those made out of 2202^{20} bytes; not the fake 10610^6-byte megabytes that storage manufacturers use.

  3. This is a pretty good failure mode: better in my opinion than letting writes silently fail, which would result in filesystem corruption, or suddenly becoming fully inaccessible. This gives the user a chance to read a full copy of the last state before failure. Kudos to Samsung for that.



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